The present invention relates to an EDM apparatus for machining a workpiece by electrical discharges by means of an electrode tool, and more particlarly the invention relates to an EDM apparatus provided with a pulse generator supplying to the machining zone between the workpiece and the electrode tool current pulses of small energy and of particular shape, for the purpose of obtaining a good surface finish on the machined surface of the workpiece.
In EDM apparatus, the electrode tool is mounted in a tool holder which is generally insulated from the rest of the apparatus. This creates a parasitical capacitance which generally results in some deterioration of the workpiece surface finish. The parasitical capacitance does not present any great inconvenience during rough machining at high material removing rates. However, in the course of finish machining, the parasitical capacitance stores electrical energy which is recovered at each ocurrence of an electrical discharge across the machining zone and which prevents machining with electrical discharges, as provided by the pulse generator, of less energy than the energy stored by the parasitical capacitance.
A variety of solutions have already been proposed for improving as much as possible the workpiece surface finish in the course of EDM finish machining. It has been, among others, suggested to disconnect the high power pulse generator required for initial rough machining of the workpiece and for high rate machining of the workpiece, and to connect across the machining zone a low power pulse generator having a low capacitance at its output. Attempts have also been made to increase as much as possible the capacitance of the cable connecting the pulse generator across the machining zone, and a variety of means have been proposed for controlling the wave form of each current pulse, more particularly for obtaining pulses having a leading front of relatively low slope.
The diverse solutions which have hitherto been proposed are only partially effective because they generally consist of compromises which do not eliminate the parasitical capacitance between the electrode tool and the remaining of the EDM apparatus structure. In order to decrease the parasitical capacitance it has been proposed to insulate not only the electrode tool holder, or machining head, from the rest of the EDM apparatus but, in addition, to insulate the workpiece itself. In such manner, a second parasitical capacitance is created, and the second parasitical capacitance is connected in series with the first parasitical capacitance such that the total capacitance is decreased, as disclosed in German Pat. No. 3,028,309. Such an arrangement results in a costly complication which, in addition, decreases the structural rigidity of the apparatus, which is a great inconvenience when it is desired to machine a workpiece with high accuracy.
Another solution, which is also complex and costly, as disclosed in German Patent No. 3,131,037, consists in partially discharging the parasitical capacitance through a shunt circuit comprising a transistor which must be controlled at high frequency and with highly accurate timing.